1. Field of the Invention
The present invention relates to a method and apparatus for selecting portions of a video signal for further processing, and more specifically, to a method and apparatus for selecting predetermined pixels, lines and frames of a video signal generated by an x-ray detector in a computed tomography (CT) imaging system.
2. Description of the Related Art
In a typical CT imaging system, a cone of x-rays are emitted by an x-ray source toward an object under test. An x-ray detector detects x-rays which pass through the object under test and generates an image frame of the detected x-ray pattern. This image frame is formatted into a video signal according to a number of conventional video formats, including NTSC, PAL and SECAM video formats. Using the NTSC format as an example, the image frame comprises 525 lines of video information, each line including, for example, 512 pixels of video information.
In an exemplary CT imaging system, an object under test is scanned for 60 seconds and 30 image frames are generated every second. These 1800 image frames are transformed by a CPU into a CT image of a slice of the object under test. Accordingly, each of the 1800 image frames of video information must be collected and digitized before being sent to the CPU for further processing.
Conventional devices for collecting image frames for digitization collect all lines of all 1800 image frames generated during the scanning period of the object under test. Digitizing this much video information is very expensive and can significantly increase the system processing time. Further, it is typically necessary to store the digitized data in a buffer memory before transferring the digitized data to the CPU system memory. These large amounts of data require large amounts of buffer memory, which further adds to the cost of the system.
Moreover, since conventional frame collectors collect and digitize every pixel of every line of every image frame generated during the scan, it is necessary to irradiate the object under test with x-rays during the entire scanning period, i.e. 60 seconds in this example. If the object under test is a human being or an animal, this x-ray dose can be very dangerous.